Rainwater harvesting is the accumulation and deposition of rainwater for reuse on-site rather than allowing it to runoff. Uses include water for the gardens, water for livestock, water for irrigation water for domestic use with proper treatment, and indoor heating for houses, etc. In many places, the water collected is just redirected to a deep pit with percolation, The harvesting water can be used as drinking water as well for storage and other purposes like irrigation.


Rainwater harvesting provides an independent water supply during regional water restriction and in developed countries is offered to supply the main supply. It provides water when there is a drought, can help mitigate flooding of low-lying areas, and reduces demand in wells which may enable groundwater levels to be sustained. It also helps in the availability of potable water as rainwater is substantially free of salinity and other salts.


The concentration of contaminants is reduced significantly by diverting the initial flow of runoff water to waste, improved water quality can also be obtained by using a floating draw mechanism(rather than from the base of the tank) and by using a series. The stored rainwater may need to be analyzed properly before use in a way appropriate to ensure its safe use. The quality

of collected rainwater is generally better than that of surface water. Contamination is always possible by airborne dust and mist, Bird feces & other debris. So some treatment may be necessary, depending on how water will be used.

System Setup:

Rainwater harvesting systems can be installed with minimal skills . The system should be sized to meet the water demand throughout the dry season since it must be big enough to support daily water consumption.

Rainwater Harvesting for those who do not have proper roof:

Rainwater can also be harvested by those who do not have proper

roof by creating a temporary collection surface by using a clean piece of cloth . Four corners of the cloth may be tied with separate threads stretched, three feet above the ground, and tied tightly to four supports (poles/ supports/ walls, etc.) during a rainy day. As the rainwater falls on the outstretched cloth a depression will be formed in the middle and all the water will get collected at the center since the cloth is porous dripping/ flowing down at the center. A can or vessel can be placed to collect this pure rainwater for further storage in an enclosed tank or a larger container for future use.

Importance of rainwater harvesting:

I. It allows individuals to play a role in intelligent rainfall management.

II. It helps in taking off the pressure of the local

aquifers and municipal water systems.

III. It provides a measure of independence to water


IV. It helps to reduce storm water runoff by

intercepting and sequestering it for later use.

V. To meet the requirements of building bye-laws


VI. To build a water-literate and prudent society


Bangladesh is a flat and low-lying country situated within the Delta formed by the Ganges and Brahmaputra rivers. The approximate annual average rainfall is 2300 mm which is almost 3 times more than of world’s annual average (800mm) rainfall. And Chittagong is a coastal area, where the rainfall rate is bigger than in many other parts of Bangladesh.

The average annual rainfall varies from a maximum of 5690 mm in the northeast part of the country to a minimum of 1110 mm in the western part. About 80% of rainfall takes place during the monsoon in Bangladesh. It is a matter of great regret that such enormous potentiality of using rainwater mostly remains unutilized.

Fig: People are gathered for pure water

The Fundamentals

Rainwater Harvesting (RWH) involves the capture, storage and use of rainwater and run-off for domestic or agricultural purposes. Essentially, RWH systems use the principle of conserving rainwater ‘when it falls’, in the process recharging groundwater.

In urban areas, rainwater can be collected from the roof, paved and unpaved areas of a house, a block of a colony, a park, a playground, parking areas schools, office complexes, lakes and tanks.

There are two ways of using the harvested rainwater, through (1) storage in receptacles, and to (2) recharge into the aquifer.


A few, basic elements are common to all RWH systems. These are as follows:

1. The catchment area where the rain falls;

2. The conveyance or conduit system that channels the flow of water in a given direction;

3. The first flush (a valve that ensures that run-off from the first spell of rain is flushed out and does not enter the system) and the filter system; and,

4. The storage area, consisting of tanks/receptacles

5. The recharge area, where the harvested rainwater is used to replenish the groundwater.

Project objective:

a) To meet the increasing demand of water.

b) To reduce the use of underground water.

c) Supplement domestic water needs.

d) To raise the underground water level to reduce groundwater pollution



Rainwater is collected from the roof via guttering and down-spouts. Its gravity-feeds through underground drainage pipe and is then passed though a filter before entering the tank. Filter can be remote from the rainwater storage tank or can be fitted inside the turret of the tank. The filter removes any leaves or larger particles from the rainwater before it is delivered into the tank. This improves the quality of the water being stored in the tank.

The rainwater enters the tank though a calmed inlet device which prevents any sediment which may build up over long periods from being disturbed and discoloring the main body of rainwater.

A submersible pump is installed inside the below rainwater tank, when the pump or controller senses a drop in pressure due to someone turning on a tap or flushing a toilet it starts to a rainwater . This rainwater then automatically delivered by the Rainwater Harvesting unit to the point of use.

(Source: Neptune aqua water solutions-2013)


1.Collecting rainwater,

2. Filtering rainwater,

3.Storing rainwater ,

4.Removing rainwater,

5.Topping up rainwater.



02. Bermudas,


04.Thomas Islands,

05.US Virgin Islands,


07.Islands of Hawaii,




While there are many ways to harvest rain water, it is usually done in one of two ways. The water is collected either from roofs or saved in underground cisterns. Roof catchment systems move the water into a storage container (sometimes underground), like a rain barrel, though pipes or gutters. Various types of rainwater harvesting systems available. The collected water can be used for watering lawns, washing cars, flushing the toilet or whatever else you can think of.


1.Catchments: The catchment of a water harvesting system is the surface which directly receives the rainfall and provides water to the system. It can be a paved area like a terrace or courtyard of a building, or an unpaved area like a lawn or open ground. A roof made

of reinforced cement concrete (RCC), galvanized iron or corrugated sheets can also be used for water harvesting.

Fig: catchment area

2.Coarse mesh:

It is present at the roof and it functions as a barrier to the passage of the debris.

Fig: coarse mesh

3. Gutters:

The channels all around the edge of a sloping roof collect and transport rainwater to the storage tank. The main functions of gutters are to collect and transport rainwater to the storage tank. The gutter can be semi-circular or rectangular and can be made using. It could be made either using galvanized iron sheets or PVC material. The size of the gutter should be according to the flow during

Highest intensity rain. It is advisable to make them 10 to 15%.

Fig: Gutters


These are the pipelines or drains that carry rainwater from the catchment or rooftop area to the harvesting system. Conduits can be of any materials like polyvinyl chloride (PVC) or galvanized iron (GI), materials that are commonly available.


The following table gives an idea about the diameter of pipe required for draining out rainwater based on rainfall intensity and roof areas.

5. First-flushing:

A first flush device is a valve that ensures that runoff from the first spell of rain is flushed out and does not enter the system. This needs to be done since the first spell of rain carries a relatively larger amount of pollutants from the air and catchment surface.

Help flush out the first2.5 mm of rainfall Filters out air-borne and

other pollutants from the catchment usually present in the first rains

First flush devices depends on variety of factors

 The condition of the catchment

 Type of catchment – Smooth or slope roofs will need less

water to wash off contaminates compared to rough or flat


 The dry period before the rain

 Area of the catchment- Small catchments require less water

Intensity and amount of rainfall

First 15-20 minutes of rainfall should be discarded as first flush



The filter is used to remove suspended pollutants from rainwater collected over roof. A filter unit is a chamber filled with filtering media such as fibre , coarse sand and gravel layers to remove

debris and dirt from water before it enters the storage tank or recharge structure. Charcoal can be added for additional filtration. When rainwater is harvested in a large rooftop area, the filtering system consists of 3 concentric circular chamber is filled with sand, the middle one with coarse aggregate and inner most layer with pebbles.

a) Charcoal water filter ,

b) Sand filters,

c) Dewas filters,

d) RO membrane filter,

e) UF filter,

f) UV filter.


When rainwater is harvested in a large rooftop area, the filtering system should accommodate the excess flow. A system is designed with three concentric circular chambers in which the outer chamber is filled with sand , the middle one with coarse aggregate and the inner-most layer with pebbles.

1.Filter channel:

One square meter in cross-section and eight m in length, laid across the tank embankment, the filter channel consists of three uniform compartments, the first packed with broken bricks, the second with coarse sand, followed by fine sand in the third compartment.